This invention relates to semiconductor electronic devices (particularly to a semiconductor electronic device structure which also includes a metal plate joined to a body of hardened resin), and to methods of making such device structures.
As is known, integrated circuits and other active electronic devices or components are formed on chips of a semiconductor material, having a surface area on the order of a few square millimeters, and require, for their connection to an external electric circuit, special supporting, enclosing and electric interconnection structures. A typical structure suited for the purpose basically comprises a polymer body enclosing a chip which is connected (by thin wires soldered to metallized regions specially provided on its surface) to corresponding electric conductors or terminal leads led out of the polymer body. With power integrated circuits (i.e. devices which are designed to operate on high currents and which are therefore liable to become heated to a significant extent), such structures also comprise a small metal plate through which a chip mounted thereon can transfer the heat generated during its operation to the environment.
In manufacturing the last-mentioned structures, the plate is blanked from sheet metal, e.g. copper, possibly along with other like plates which are held together by sections of the sheet metal adapted to be shorn off at a later processing stage; thereafter, the chip is secured on the metal plate either by soldering with a low-melt alloy such as a lead-tin alloy, or by cementing with a suitable adhesive such as an epoxy adhesive; a set of metal strips, intended to become the terminal leads for the device, are then blanked from thin sheet metal but still left joined to one another by interconnecting sections, and mounted to the plate in an electrically insulated manner therefrom; thin wires, usually of gold, are bonded, on the one side, to the metallized regions of the chip using a low-melt alloy, and on the other side, welded to the ends of the metal strips using a so-called "thermosonic" process wherein heat and ultra-sound are applied simultaneously; thereafter, the assembly is placed into a specially provided mold, into which a polymer material such as a thermosetting epoxy resin is then introduced in a liquefied state; on curing the resin, a structure is obtained which comprises a solid polymer body (of hardened resin) encapsulating the above-described elements excepting for one face of the metal plate and part of the metal strips, i.e. of the device terminal leads, and their interconnecting sections; the latter are then removed along with any interconnecting sections between plates (e.g. by blanking) to yield the finished electronic product.
A typical problem with the above-described structures is their low reliability from insufficient adhesion of the resin body to the metal parts. It has been verified that a large proportion of the failures occurring to such devices originates from moisture entering the body through interstices which result from separation of the hardened resin from some surface areas of the heat-sink plate and the terminal leads. This separation is to be attributed basically to shear forces developing between the metal and the polymer during the cooling step which follows the molding operation and during normal thermal cycles of operation owing to the difference between the thermal expansion coefficients of metal and polymer.
To solve this problem at least in part, a technique is known whereby the surface and/or edges of the heat-sink plate are machined to form grooves and/or undercut portions therein which can allow the liquefied resin to flow therealong during the injection molding operation, thereby providing not only improved anchoring of the polymer body on the plate but also improved imperviousness to moisture after the resin has cured. This expedient has proved effective to improve stability of the mechanical bond between the polymer body and the heat-sink plate, but has been less effective in reliably providing a hermetic seal: since the action of this expedient primarily affects the interface between the polymer and the heat sink, the seal across the polymer/leads interface is not improved.
The disclosed innovations advantageously provide a semiconductor electronic device structure (of the type which provides heat-sinking and encapsulation for the electronic device by use of a metal plate joined to a body of hardened resin) which can afford improved overall reliability, and moisture-tightness in particular.
This object is achieved, according to the invention, by the provision of a structure wherein the metal plate and/or the end portions of the terminal leads inside the polymer body exhibit a higher roughness of their surfaces in contact with the polymer body than 1 (R.sub.a =1 .mu.m). (The parameter R.sub.a, which provides a quantitative measure of surface roughness, is conventionally defined as ##EQU1## where y=f(x) is a function representing the profile of a surface to be measured at the intersection line of the surface with a plane perpendicular thereto and considered between two points x.sub.1 and x.sub.2. The unit of measure is the micron (10.sup.-6 m).) This is preferably achieved by fabricating these metal elements using selectably roughened dies.
According to certain embodiments of the invention, there is provided: a method for fabricating packaged electronic device structures, comprising the steps of: affixing a chip, which contains one or more miniature electronic devices, to a metal plate of which substantial portions have a surface roughness greater than about R.sub.a =1, and other portions have a surface roughness less than about R.sub.a =1; attaching a lead frame to said plate, said lead frame comprising multiple flat metal strips joined in a substantially electrically continuous sheet, substantial portions of said lead frame having a surface roughness greater than about R.sub.a =1; electrically connecting predetermined portions of said chip to leads of said lead frame; encapsulating said chip, said wires, portions of said plate, and portions of said lead frame in an insulating material; and trimming outer portions of said lead frame to provide electrically separated leads connected to said bond pads and which protrude from said body.
According to certain embodiments of the invention, there is provided: a method for fabricating packaged electronic device structures, comprising the steps of: blanking sheet metal, using a die which has at least some portions thereof artificially roughened, to produce a metal plate of which substantial portions have a surface roughness greater than about R.sub.a =1, and other portions have a surface roughness less than about R.sub.a =1; affixing a chip which contains one or more miniature electronic devices, and one or more electrical bond pads, to said metal plate; attaching a lead frame to said plate, said lead frame comprising multiple flat metal strips joined in a substantially electrically continuous sheet; bonding to form wire connections from said bond pads of said chip to leads of said lead frame; encapsulating said chip, said wires, portions of said plate, and portions of said lead frame in an insulating material; and trimming outer portions of said lead frame to provide electrically separated leads connected to said bond pads and which protrude from said body.
According to certain embodiments of the invention, there is provided: a method for fabricating packaged electronic device structures, comprising the steps of: blanking and coining sheet metal to produce a metal plate; blanking and coining sheet metal, using a die which has at least some portions thereof artificially roughened, to produce a metal lead frame of which substantial portions have a surface roughness greater than about R.sub.a =1, said lead frame comprising multiple flat metal strips joined in a substantially electrically continuous sheet; attaching said lead frame to said plate, said lead frame comprising multiple flat metal strips joined in a substantially electrically continuous sheet; affixing a chip which contains one or more miniature electronic devices, and one or more electrical bond pads, to a metal plate; bonding to form wire connections from said bond pads of said chip to leads of said lead frame; encapsulating said chip, said wires, portions of said plate, and portions of said lead frame in an insulating material; and trimming outer portions of said lead frame to provide electrically separated leads connected to said bond pads and which protrude from said body.
According to certain embodiments of the invention, there is provided: a packaged electronic device structure, comprising: a plate of thermally and electrically conductive material; a microelectronic device chip having a first surface thereof electrically and thermally connected to said plate, and containing one or more miniature electronic devices and one or more bond pads on a second surface thereof; a plurality of externally accessible conductive leads, each having one end adjacent to said chip; wires interconnecting said leads to said bond pads of said chip; and a polymer body encapsulating said chip, and each of said wires, and at least part of said plate; wherein at least some portions of said plate have a surface roughness of greater than about R.sub.a =1, and other portions have a surface roughness less than about R.sub.a =1; and wherein at least some portions of said leads have a surface roughness of greater than about R.sub.a =1.
According to certain embodiments of the invention, there is provided: a packaged electronic device structure, comprising: a plate of thermally and electrically conductive material; a microelectronic device chip electrically and thermally connected to said plate; a plurality of externally accessible conductive leads, each having one end adjacent to said chip; wires interconnecting said leads to said chip; and a polymer body encapsulating said chip, and each of said wires, and at least part of said plate; wherein at least some portions of said plate have a surface roughness of less than about R.sub.a =1; and wherein at least some portions of said leads have a surface roughness of greater than about R.sub.a =1.
According to certain embodiments of the invention, there is provided: a packaged electronic device structure, comprising: a plate of thermally and electrically conductive material; a microelectronic device chip electrically and thermally connected to said plate; a plurality of externally accessible conductive leads, each having one end adjacent to said chip; wires interconnecting said leads to said chip; and a polymer body encapsulating said chip, and each of said wires, and at least part of said plate; wherein some portions of said plate, including at least 5% of the surface area thereof, have a surface roughness of greater than about R.sub.a =1, and other portions of said plate have a surface roughness of about R.sub.a =1 or less.
According to certain embodiments of the invention, there is provided: a semiconductor device structure comprising a metal plate with support and heat dissipation functions, a chip of a semiconductor material attached to said plate, terminal leads for connection to external electric circuitry, and a polymer body encapsulating said metal plate to leave at least a surface portion thereof, said chip of semiconductor material, and end portions of said leads exposed, wherein at least predetermined areas, in contact with said polymer body, of said metal plate and said end portions of said leads exhibit a higher roughness than 1 (R.sub.a &gt;1 .mu.m).